Steel is an important structural material, but unless covered with a protective coating, it is notoriously prone to rusting and other types of corrosion, even from contact with the ambient natural atmosphere and more rapidly from many other types of environments to which articles made of steel are likely to be exposed. Accordingly, steel articles are commonly protected from corrosion by providing them with some kind of protective coating, such as paint.
One of the most durable and effective protective coatings for steel is vitreous enamel, which is alternatively called xe2x80x9cporcelain enamelxe2x80x9d or simply xe2x80x9cenamelxe2x80x9d. Ordinarily, such enamel, like most other protective coatings, will not bond effectively to clean steel, so that an intermediate layer is needed in order to obtain an enamel coating with good adhesion and durability. A thin layer of nickel or zinc has been generally used in the prior art as an intermediate bonding layer for vitreous enamel. This is relatively costly in process time, chemicals, and/or equipment and often does not provide as good adhesion as would be desirable. Accordingly, a major object of this invention is to provide a process for forming a high quality corrosion-protective coating, particularly a vitreous enamel coating, on a steel substrate, said process not requiring any use of an intermediate metallic layer between the steel and the protective coating and achieving at least one of the following benefits over the prior art processes utilizing a metallic intermediate layer: lower materials cost; reduced process time; reduced requirements for energy input into the process; lower cost capital equipment requirements for the process; better adhesion of the protective coating to the steel substrate, especially under moderate flexural forces; and better resistance of the protective surface formed to corrosion or other damage from environments with which it comes into contact. An alternative object is to provide novel compositions that are advantageously used in a process according to the invention.
Except in the claims and the operating examples, or where otherwise expressly indicated, all numerical quantities in this description indicating amounts of material or conditions of reaction and/or use are to be understood as modified by the word xe2x80x9caboutxe2x80x9d in describing the broadest scope of the invention. Practice within the numerical limits stated is generally preferred, however. Also, throughout the description, unless expressly stated to the contrary: percent, xe2x80x9cparts ofxe2x80x9d, and ratio values are by weight or mass; the term xe2x80x9cpolymerxe2x80x9d includes xe2x80x9coligomerxe2x80x9d, xe2x80x9ccopolymerxe2x80x9d, xe2x80x9cterpolymerxe2x80x9d and the like; the description of a group or class of materials as suitable or preferred for a given purpose in connection with the invention implies that mixtures of any two or more of the members of the group or class are equally suitable or preferred; description of constituents in chemical terms refers to the constituents at the time of addition to any combination specified in the description or of generation in situ within the composition by chemical reaction(s) noted in the specification between one or more newly added constituents and one or more constituents already present in the composition when the other constituents are added, and does not preclude unspecified chemical interactions among the constituents of a mixture once mixed; specification of constituents in ionic form additionally implies the presence of sufficient counterions to produce electrical neutrality for the composition as a whole and for any substance added to the composition; any counterions thus implicitly specified preferably are selected from among other constituents explicitly specified in ionic form, to the extent possible; otherwise such counterions may be freely selected, except for avoiding counterions that act adversely to an object of the invention; the word xe2x80x9cmolexe2x80x9d means xe2x80x9cgram molexe2x80x9d, and the word itself and all of its grammatical variations may be used for any chemical species defined by all of the types and numbers of atoms present in it, irrespective of whether the species is ionic, neutral, unstable, hypothetical, or in fact a stable neutral substance with well defined molecules; the terms xe2x80x9csolutionxe2x80x9d, xe2x80x9csolublexe2x80x9d, xe2x80x9chomogeneousxe2x80x9d, and the like are to be understood as including not only true equilibrium solutions or homogeneity but also dispersions that show no visually detectable tendency toward phase separation over a period of observation of at least 100, or preferably at least 1000, hours during which the material is mechanically undisturbed and the temperature of the material is maintained within the range of 18-25xc2x0 C.; the first definition of an acronym or other abbreviation applies to all subsequent uses of the same acronym or other abbreviation; and the term xe2x80x9cpaintxe2x80x9d and its grammatical variations includes all similar types of coatings that may be described by more specialized names such as xe2x80x9clacquerxe2x80x9d, xe2x80x9cvarnishxe2x80x9d, xe2x80x9cprimer coatxe2x80x9d, xe2x80x9ctop coatxe2x80x9d, or the like.
It has surprisingly been found that an excellent adhesion of vitreous enamel to steel can be achieved via an intermediate layer formed on the steel by bringing it into contact with a liquid composition that contains water and a component of solute selected from the group consisting of dissolved silicates and dissolved borates and, optionally, surfactant. The modification of the steel surface achieved by treating it with silicates and/or borates is also effective for improving the adhesion of steel to other protective type coatings such as paint.